1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a method of controlling a top margin to uniformly regulate a position of an image on a printing medium, and an image forming apparatus employing the method.
2. Description of the Related Art
A top margin of a printing medium refers to a non-image area on the printing medium. More specifically, the top margin refers to a portion from a leading end to a position where an image starts to be transferred. Therefore, control of the top margin of the printing medium refers to the control of the transferring position of the image on the printing medium.
FIG. 1 is a sectional view schematically illustrating a conventional color image forming apparatus. Referring to FIG. 1, the color image forming apparatus comprises four photoconductive drums 10 whereon electrostatic latent images are formed, a laser scanning unit 12 to generate the electrostatic latent images on the respective photoconductive drums 10, four developing units 14 to change the electrostatic latent images into visible images, four transferring units 16 to transfer the visible images onto a printing medium, and a conveying belt 18 disposed between the photoconductive drums 10 and the transferring unit 16 to move the printing medium.
In the conventional image forming apparatus, upon receiving a printing command from a host device such as a computer, a controller (not shown) drives a pickup roller 20 to pick up the printing medium from a paper feeding cassette 24 or a multifunction paper feeding unit 26. The picked-up printing medium is conveyed between the conveying belt 18 and the photoconductive drums 10 by a feeding roller 22 at a constant velocity so as to have an image transferred thereupon. While being fed, the printing medium is detected by a registration sensor 28 mounted on a printing medium feeding path before entrance to the conveying belt 18. Also, the laser scanning unit 12 exposes the photoconductive drums 10 to a laser beam after a preset time from a point of detecting the printing medium. The preset time is determined as a difference between a time obtained through a distance from the registration sensor 28 to a transfer position TP and a feeding velocity of the printing medium, and a time taken from a laser-scanning position to the transfer position TP. Therefore, a setup of a transfer location on the printing medium depends on the predetermined feeding velocity and feeding distance of the printing medium.
However, since the pickup roller 20 or the feeding roller 22 can be abraded or stained with impurities, when the transfer position TP is determined as explained above, radiuses of the rollers 20 and 22 may change, thereby generating an error in the feeding velocity. Also, slips between the printing medium and the rollers 20 and 22 may cause an error in the feeding velocity of the printing medium. Due to the error, the transfer position TP for the image on the printing medium may be incorrectly determined. Furthermore, in a case in which the error is considerable, some images may miss being transferred onto the printing medium, thereby causing a loss of the information.